Hexagonal synthetic corundum (synthetic sapphire) is excellent in terms of hardness, light transmission, and resistance to chemicals, and hence is used in a flow cell, for example, which is incorporated in a particle counter which count particles contained in a liquid such as hydrofluoric acid, for example.
Manufacturing products of synthetic corundum needs to join synthetic corundum pieces that have been cut to predetermined dimensions. However, since single crystals such as synthetic corundum pieces have different coefficients of thermal expansion dependent on the direction in the crystals, synthetic corundum pieces that have simply been joined together would tend to peel off, and are not suitable for use in flow cells through which the hydrofluoric acid or the like passes.
There has been known a method of manufacturing a structural body made of a single crystal of integral synthetic corundum as disclosed in Japanese patent publication No. 5-79640. According to the disclosed method, an ingot of a single crystal of synthetic corundum is cut into a first prism, and a surface of the first prism is optically ground to a flatness accuracy that is equal to or less than λ/8 of the wavelength λ(=6328 Å) of red light, thus producing a second prism. Four surfaces, including the optically ground surface, of the second prism are surrounded by a jig, and cut into a first planar piece with a plane perpendicular to the optically ground surface. Then, both upper and lower surfaces of the first planar piece are optically ground to produce a second planar piece, which is cut into cut planar pieces. The cut planar pieces are separated and superposed by an assembling jig such that their upper pieces are separated and superposed by an assembling jig such that their upper and lower optically ground surfaces are aligned with each other for aligned crystalline planes, ridges, axes, and axial angles. A small pressure is applied to the planar pieces to completely eliminate any interference fringes on their transparent boundary surfaces to chemically pressurize and join the planar pieces into intimate contact with each other. The planar pieces are then heated at a temperature of 1200° C. which is lower than the melting point of 2030° C. of synthetic corundum, so that they are joined into close contact with each other.
The flatness is defined as follows: When a planar body (P) is sandwiched by two geometrically parallel planes, the flatness is expressed by the distance (f) between the two parallel planes which is minimum, and represented by a flatness X mm or X μm (X is a numerical value). Therefore, the flatness accuracy that is equal to or less than λ/8 means the flatness of at most 0.0791 μm.
According to the above method of manufacturing a structural body made of a single crystal of integral synthetic corundum, a reference surface is formed on the first prism by being optically ground to a flatness accuracy that is equal to or less than λ/8 of the wavelength λ (=6328 Å) of red light, i.e., a flatness of at most 0.0791 μm. Then, a planar piece having the reference surface is cut from the first prism, and further cut into a plurality of planar pieces, which are combined together with respect to their reference surfaces so as to be superposed for aligned crystalline planes, ridges, axes, and axial angles. When a small pressure is applied to the planar pieces to completely eliminate any interference fringes on their transparent boundary surfaces, the planar pieces are chemically pressurized and joined in intimate contact with each other, not just held in intimate contact with each other. The planar pieces are then heated at a temperature of 1200° C. which is lower than the melting point of 2030° C. of synthetic corundum, so that they are joined into a structural body made of a single crystal of integral synthetic corundum where no boundary surfaces are present between the planar pieces and the planar pieces will not peel off.
In order to produce a synthetic corundum cell according to the above method, it is usually necessary to cut a cylindrical ingot into a prism, grind a surface of the prism to a highly small flatness of 0.0791 μm, grind planar pieces cut from the prism to a highly small flatness of 0.0791 μm, and manage a particular temperature of 1200° C. Consequently, the production process is complex, inefficient as it requires very high grinding process, and needs difficult process control. The production process is not practical, and the cost of synthetic corundum cells manufactured by the method is extremely high.
The inventor has made research efforts to develop a practical technology for joining synthetic corundum through a simple process even with a certain large value of flatness, unlike the above conventional unpractical technique. The inventor attempted to superpose synthetic corundum pieces whose ground surfaces are fully held in optical contact and heat them while adjusting the heating temperature. However, this method not only produced boundaries at the joined surfaces, but also formed smears in the boundaries, resulting in a failure to meet optical requirements. After repeating trial-and-error efforts, the inventor found that a structural body which has boundaries, but contains highly reduced smears and meet optical requirements, and are practical in terms of mechanical strength, though not firm enough to be integral, can be produced by keeping only ends of joined surfaces of synthetic corundum pieces in optical contact with each other and heating them, rather than the conventional common technical approach to superpose two optical members whose entire joint surfaces are held in optical contact with each other.
The present invention has been made in view of the problems of the conventional method of manufacturing a structural body made of a single crystal of integral synthetic corundum. It is an object of the present invention to provide a synthetic corundum cell having desired optical characteristics and mechanical strength which is produced efficiently in a simple manufacturing process.